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Old 01-30-2013, 12:50 PM   #29
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Quote:
Originally Posted by Shankenstein View Post
f_nat1 = constant * sqrt(spring rate / sprung mass)
What is the purpose of the constant in the natural frequency calculations? I haven't seen that before.

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Tire rate / spring rate < sprung mass / unsprung mass
Can you explain how you get to this conclusion from what you presented above? You can use mathematics/vibrations if you'd like.
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Old 01-30-2013, 01:56 PM   #30
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As stated, we have a highly intelligent crowd for peer-review. Thanks for signing up to make a post, Josh.

I pulled most of the info from a guide found here: LINK to PDF

First point. Here's the traditional formula for a spring-mass oscillator:
F = sqrt(spring constant / mass)

It will output in rad/s. Radians are wondeful for Bode plots, but not good for normal people... so 1/2/pi = 0.159 is the conversion factor for Hz.

Second point may be very valid. It's a fine handwaving argument, but it may not be correct. I picked that up from a random forum post on the miata forums. Let's find out:

Tire dynamics should minimally affect suspension dynamics.
A decade of frequency separation is considered sufficient.
F_tire > 10 * F_susp
sqrt(tire rate / unsprung mass) > 10 * sqrt(spring rate / sprung mass)
If both values are more than one,
tire rate / unsprung mass > 100 * spring rate / sprung mass
tire rate / spring rate > 100 * unsprung mass / sprung mass

for our example:
6500 / 131 > 100 * 83 / 618
49.6 > 13.43 --> sufficiently separated

I guess I should amend the above statement. Thanks for pointing it out!

Continuing this thought:
If we calculate the max spring rate that can be used without being affected by tire dynamics (at stock pressures):
max front wheel rate = 484 lbs/in
max front spring rate = 526 lbs/in
max rear wheel rate = 422 lbs/in
max rear spring rate is = 548 lbs/in

At autox pressures, max spring rates would be 809 (front) and 843 (rear). In metric, that's 14.2k and 14.8k. Interesting, not that anyone would want to run them that stiff anyways.

Last edited by Shankenstein; 01-30-2013 at 02:33 PM.
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Old 02-02-2013, 01:01 PM   #31
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Old 02-02-2013, 01:41 PM   #32
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New discussion point: Sway bars!

Roll center (according to SAE) - The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll.

Layman's definition - This is a neutral point for your suspension. Applying lateral force at this height will generate no roll (vertical movement at either corner).

Let's consider a 1.0 g turn in a 2645 lbs car. That means ~2645 lbs force will be applied to the center of mass/gravity. Here's an illustration:


4 possible options:

1) Roll center height = center of gravity
There is no roll. If there is sufficient grip in the tires, your car will turn like a go-kart or a door-hinge (flat). This does sent alot of force through the control arms, and the spring/damper are not used at all.

2) 0 < roll center height < center of gravity
There will be a moment (torque) generated, since the lateral force is applied at a different height than the reaction force.

Torque = Force * distance
Roll Torque = 2645 * abs(center of gravity height - roll center height)

Since there is a torque, there will be reaction forces. Typically this duty falls on the springs and sway bars. Most race cars try to keep the roll center height at 15-30% of the center of gravity height.

3) roll center height = ground height
The control arms won't be loaded, and all forces will be sent through the spring/damper. Not horrible, just sub-optimal.

4) roll center height < ground height
The spring/damper will see an amplified force, and can cause the control arms to see the wrong type of force (compression vs tension). This isn't necessarily bad, but I can't recommend ever having an underground roll center, unless you overbuild the spring/damper to compensate for it.

TL;DR - Try to keep the roll center between the center of gravity and the ground. Lower is better, but don't go underground.
Didn't see this got stickied. I applaud your efforts. I want to develop a full system model, including a driver, using state space equations so you can change the initial conditions easily for simulation purposes. The problem is I hate matrices lol.

Be careful with roll centers. Over the last few years, it has become one of the most misunderstood terms on the internet. You are using the proper, SAE definition, but you are giving 2D, kinematic scenarios of roll center height. What we need to determine are the force application points (FAP) and use the force-based roll center, not the kinematic roll center.

The KRC tells you an arbitrary point in space. It only works in a symmetrical 2D case. As soon as you turn the wheel and the suspension begins to compress/decompress as the car begins to roll, the model is no longer valid. So the KRC only works when the car is static and what good does that do?

Worrying about roll center migration is also a load of crap. If the roll center can migrate up-and-down and side-to-side, what happens when the roll center is outside the wheelbase? All four corners would simultaneously be in tension or compression, a physical impossibility.

Mitchell himself discusses the KRC vs FBRC here: http://www.neohio-scca.org/comp_clin...namics2007.pdf

There is nothing wrong with having a KRC below ground per se. Sometimes, such as in the case of F1, there's a below ground roll center, but that's because suspension as a whole is compromised around the aero. But a below ground roll center means the suspension has anti-jacking built into it, creating a more stable aero platform in return. If you are going to discuss roll center height, you must include jacking/anti-jacking forces as a result.
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Old 02-02-2013, 02:39 PM   #33
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Preface: I no engineering background.

I get that you're basically trying to develop a model of the car's suspension. Once you have this created, what kind of information can we learn from this? How does this benefit the owners of these cars?

If I'm understanding this correctly, you're using a simplified version of this car (model) in order to get a better understanding of this car's vehicle dynamics, correct?
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Old 02-02-2013, 02:46 PM   #34
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force application points (FAP)
hehehehe
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Old 02-02-2013, 02:57 PM   #35
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Quote:
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Preface: I no engineering background.

I get that you're basically trying to develop a model of the car's suspension. Once you have this created, what kind of information can we learn from this? How does this benefit the owners of these cars?

If I'm understanding this correctly, you're using a simplified version of this car (model) in order to get a better understanding of this car's vehicle dynamics, correct?
I suspect that many people will find many different uses for a suspension model. I know that this information is really beneficial to me as I try to input the correct variables into this suspension dynamics calculator. It should be useful in finding theoretically ideal spring rates and damper settings (either through re-valving or adjustments), among other things that will maximize the car's potential grip and performance.

Unfortunately, the accuracy of that data will be limited by the accuracy of the inputs, and most of the input data can only be determined from having a very precise model of the car's suspension. (And fancy software )
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Old 02-02-2013, 03:24 PM   #36
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I suspect that many people will find many different uses for a suspension model. I know that this information is really beneficial to me as I try to input the correct variables into this suspension dynamics calculator. It should be useful in finding theoretically ideal spring rates and damper settings (either through re-valving or adjustments), among other things that will maximize the car's potential grip and performance.

Unfortunately, the accuracy of that data will be limited by the accuracy of the inputs, and most of the input data can only be determined from having a very precise model of the car's suspension. (And fancy software )
I wonder if damper companies (Bilstein, Koni, Eibach, etch) go into this much detail when developing their dampers.
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Old 02-03-2013, 12:40 AM   #37
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@ EarlQ,

All of the suspension nerds on here should read that paper. Not just because of the FAPs and jacking forces, either. Mitchell is the man, and he has a way of elegantly poo-pooing on everything you've ever learned.

My interpretation of roll center isn't horrifically wrong, but it isn't terribly useful either. Hopefully it will atleast serve as an elementary-level understanding for the next-level theory and kinematic simulations.

@ Ninjin,
A model is just that, a simplified and solvable interpretation of reality. A good model is reproducible and transferable. My sincere hope is that we can make something that anybody can validate for themselves and apply to their specific modification.

Ex: Somebody has heard that RCE Yellow lowering springs are the bee's thorax. They want to buy some, but would feel more comfortable knowing what the rear camber curves look like (since that's not factory adjustable). If we validate the base model, he can drop in the new spring rate and height... then be reasonably confident in the modest tire wear rates. Similarly, they could test the eccentric bushings, and see if that would correct the problem (before buying and installing them).

@ Ayau,
They have modelling, but when you're ballin' enough... you get big boy tools. 7 post rigs like this:

and shock dynos like this:

Even if it's a technician running the test and one experienced engineer calling the shots ... you can get a great setup. KONI and the rest have team of people on this, since it's their bread and butter. Just so it's said, 4 of the "posts" are the wheels, and the remainder are for the chassis.
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Old 02-03-2013, 01:54 PM   #38
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I've started a spreadsheet of the suspension coordinates.

LINK to Google Docs Spreadsheet

Very few points are correct yet. The front has been approximated based on pictures and specifications. The rear has not been configured, and is simply the program defaults. I'll fill these in shortly.

I'll leave this publicly viewable. If anybody wants "edit" capabilities, just PM me.
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Old 02-03-2013, 04:20 PM   #39
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That is awesome.
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Old 02-12-2013, 12:44 PM   #40
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For people who track their cars: if you have data logs or track maps, can you please PM me? I want to build, verify, and correlate a few different simulations.
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Old 02-15-2013, 02:51 PM   #41
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Ran across the CAD drawing for the AST 4150 (front right) strut. For the sake of building our database of dimensions, some reverse engineering was done. This data is provided for non-commercial use. If you try to rip off AST (one of the premier suspension manufacturers), expect their lawyers to sue you, the 86 community to shun your products, and numerous internet photographs of your product being teabagged in public places. Save us (and our giblets) the trouble, and just don't do it.

Distance between strut bolts is 60.5 mm --> 2.4"
Thickness of lower flange is 25.4 mm --> 1"
Distance from strut center to lower bolt is 60.7 mm --> 2.4"
Length from lower bolt to upper mount top is 392 mm --> 15.4"
Upper thread is a M12x1.25-25, with 5 mm of thread relief
Diameter of damper rod is 22 mm
Diameter of spring perch is 60 mm
Length of spring available is 181 mm --> 7.1"
Distance from lower bolt to sway bar mount is 115 mm --> 4.5"
Distance from strut center to sway bar mount is 50 mm --> 2"
Diameter of sway bar mount is 10.2 mm --> 0.4"



Edit: Main post updated with only the relevant parameters!
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Old 02-16-2013, 04:04 PM   #42
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Shankenstein - this is great. Thank you for starting it.

What products are you aiming to model first to work on your own personal suspension goals? Just curious.
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